U.S. Senate Democrats blocked an attempt by Republicans on April 27 to prevent future purchases of Iranian "heavy water" by the U.S. Energy Department.

Republicans offered an amendment to a department spending bill to bar future purchases after an $8.6 million deal to buy 32 metric tons of heavy water from Iran was announced on April 22 to help Tehran carry out its obligations under a nuclear deal with world powers.

But Democrats used parliamentary tactics to block the amendment, calling it a "poison pill" that would draw a veto from President Barack Obama.

There are no current plans for further U.S. purchases of heavy water. The heavy water purchased last week will be stored at the Oak Ridge National Laboratory in Tennessee and then resold on the commercial market for research purposes.

Heavy water is not radioactive, but has research and medical applications and can also be used to produce weapons-grade plutonium.

Under the nuclear deal, Iran is allowed to use heavy water in its modified Arak nuclear reactor, but must sell any excess supply of both heavy water and enriched uranium on the international market.